Friend of mine. Slim framework not updated anymore. Cannot make it work in recent php versions, so a fragment of a big corp needs to be maintained and isolated. I know, happens everywhere
If you mean the framework named Slim, its last point release, according to their web site, was less than a month ago, and it says that the minimum PHP version requirement is 7.2. It's possible your friend isn't able to upgrade the application to a supported version of Slim?